The dynamic load circuit of the invention finds particular utility in the power supply for a traffic or pedestrian crossing signal, or for other types of signals, for attenuating the effects of leakage currents when a particular signal is switched to its off state.
As is well known, pedestrian traffic signals usually comprise a first luminescent tubular lamp which is energized to indicate a "WALK" signal and a second luminescent lamp which is energized to indicate a "DONT WALK" signal. These lamps are alternately switched on and off, usually by solid state switches, such as triac switches. However, such switches exhibit leakage currents. Such leakage currents are usually of the order of 20 mils, which do not result in appreciable voltages except when fluorescent or neon lamps are used.
It is usual for pedestrian and traffic lights to use "green" monitors which sense a condition where two "walk" or "go" signals are energized at the same time at the same inter section for interesecting streets. Such a condition could cause a disaster, and it is a function of the "green monitors" to switch the lights to their flashing mode should that condition occur.
When neon or fluorescent tubes are used in the pedestrian or traffic lights, the voltages generated by the leakage current through the triac switches are sufficient to activate the "green" monitors which mistakenly react thinking the particular circuit is on, when it actually is off.
The dynamic load circuit of the present invention is such that when the triac switches are off, the leakage current sees a relatively low impedance to ground so that no excessive voltage builds up. However, when the triac switches are on, and the input voltage ramps up, the impedance of the dynamic load circuit immediately increases, effectively taking the load circuit out of the main circuit.